Linear variable differential transformer



1386- 9 J. cHAss LINEAR VARIABLE DIFFERENTIAL TRANSFORMER Filed Dec. 27, 1968 A /w mfl Q III I INVENTOR J'A cos C/MJS K izdfll 7M ATTO R N EY5 United States Patent 3,546,648 LINEAR VARIABLE DIFFERENTIAL TRANSFORMER Jacob Chass, Forest Hills, N.Y., assignor to Pickering & Company, Inc., Plainview, N.Y., a corporation of New York Filed Dec. 27, 1968, Ser. No. 787,400 Int. Cl. H01f 21/06 US. Cl. 336-136 4 Claims ABSTRACT OF THE DISCLOSURE A nonsymmetrical differential transformer having a null position at one end and an output voltage which is a linear function of the displacement of an armature core from the null position.

BACKGROUND OF THE INVENTION Differential transformers of the type composed of tubular bobbin of nonmagnetic, nonconductive material and having primary and secondary coils wound thereon with a displaceable magnetic armature core, the position of which determines the number and position of secondary coils magnetically coupled with the primary coil are well known. In transformers of this type secondary windings -a minimum or substantially zero output it is saidthat the null position has been achieved.

Constructions of such transformers wherein displacement of the armature in either direction from the null position results in an output voltage the amplitude of which is a function of the displacement distance irrespective of direction are known as symmetrical differential transformers whereas constructions having the null at one end providing an increased output voltage amplitude resulting from movement of the armature in one direction only are known as non-symmetrical differential transformers. Certain advantages and benefits are achieved in non-symmetrical transformers as will appear by reference to US. Letters Pats. 3,017,590, 3,031,633 and 3,376,533.

The differential transformers shown in these patents have greater utility, however, in the first, the primary coils are sectioned and complicated secondary windings are provided, while in the second, which is much simpler in construction, the secondary windings are longitudinally displaced from the primary winding. The third of these patents describes a small, relatively high power output linear transformer in which the armature travel is short but highly effective and provides good resolution. In that patent there are two discrete secondary windings, placed side by side and the primary winding is wound over the lengths of the two secondary coils. Each of the windings has a uniform number of turns however one of the secondary windings has a greater density of turns than the other. The axial dimension of the armature core is related to the coils to achieve null at an end position of the armature.

The desirability of utilizing windings of gradual varying turn density rather than discrete windings in certain applications is known.

The present design provides a low null voltage, linear 3,546,648 Patented Dec. 8, 1970 output versus displacement, a long displacement range for a given assembly length and low manufacturing cost.

SUMMARY OF THE INVENTION A differential transformer comprising a tubular bobbin of a nonmagnetic, nonconductive material having first and second secondary coils of electrically conductive wire wound around the bobbin over the entire length of the transformer and connected together in bucking series relationship, and having a primary coil of electrically conductive wire wound around the bobbin over the entire length of the transformer, and having means insulating the windings from one another, with the primary coil and one of said secondary coils having a uniform number of turns with the remaining secondary coil having a gradually decreasing turn density from one end of the transformer to the other, and an armature core movably disposed and having a length less than the axial dimension of the transformer and to permit the secondary coils to be magnetically coupled to the primary coil through opposite ends of the armature to achieve a null when the core is disposed at one end of the transformer.

DESCRIPTION OF THE DRAWINGS In the accompanying drawings:

FIG. 1 is a longitudinal sectional view of the differential transformer of the present invention; and

FIG. 2 is a circuit diagram of the differential transformer of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT A differential transformer constructed in accordance with the teachings of this invention is shown in the figures comprising an elongated primary coil 10 having a uniform number of turns per unit length, a first secondary coil 12 extending over the entire length of the primary coil and having a uniform number of turns per unit length, a second secondary coil 14 extending over the entire length of the primary coil and having a winding distribution of gradually decreasing turn density from one end to the other, and an armature core 16 of magnetic material movably disposed within the coils.

The secondary coils are connected in series bucking arrangement. The number of turns and lenth of these coils are designed to provide an electrical balance or null when the core 16 is at the end of the assembly as indicated in the figures by the numeral 18 and an electrical voltage is applied between primary input terminals 20 and 22. The output voltages of the two secondary coils in the null position are equal.

When the core is displaced from this balanced or null point the output voltage of the uniform secondary coil 12 will remain constant while the output of the second secondary coil 14 will decrease and an electrical output will appear between the secondary output terminals 24 and 26 which is a linear function of the displacement.

In operation therefore, when an input current is placed across primary coil 10 by the application of voltage between the terminals 20 and 22, it will generate a magnetic flux which encircles through the core and the windings of coils 12 and 14 which are connected in opposition. The number of turns and length of these coils are designed to provide an electrical balance when the armature is at the end of the assembly indicated by the numeral 18. When the core is displaced away from the balance point, an electrical output will result which is a linear function of the displacement. Thus, there is provided a non-symmetrical differential transformer which has its null position adjacent one end of the differential transformer and in which movement of the armature core from its null position toward the other end of the differential transformer provides an output current from the differential transformer.

Referring to FIG. 1, there is shown a preferred form of the differential transformer of the present invention.

The differential transformer comprises a tubular bobbin 30 formed of a nonmagnetic, nonconductive material such as a plastic or ceramic. Bobbin 30 has a pair of radially extending end flanges 32 and 34 of equal diameter which form a compartment.

The first secondary inductance coil 12 which is a helically wound coil of insulated electrically conductive wire is wound around the bobbin 30 within the compartment defined by flanges 32 and 34 and is of uniform turn density throughout its length. Secondary coil 14 which is also a helically wound coil of insulated electrically conductive wire is wound around the bobbin 30 between flanges 32 and 34 and is wound to have gradually decreasing turn density from right to left in FIG. 1.

The primary inductance coil 10 which is also a. helically wound coil is wound over the secondary windings 12 and 14 and throughout the length of the bobbin compartment defined by the flanges. The primary winding is also of uniform turn density.

The windings are insulated from each other and in the present embodiment the electrical conductive wire forming the transformer windings has an insulation cover ing as is known in the art.

The secondary coils 12 and 14 are connected in bucking series arrangement as shown diagrammatically in FIG. 2. The secondary coils are preferably made up of the same size wire so that they will expand or cntract the same amount and will be stressed the same amount upon changes in temperature. Also the electrical resistance of the secondary windings will change the same upon changes in temperature so that the differential transformer is electrically stable.

Armature core 16 is movably disposed within the bobbin 30. Armature core 16 comprises a rod of magnetic material which is of a length shorter than the total length of the winding 10. A sleeve 40 of a magnetic material is placed around the bobbin and discs 42 and 44 of a magnetic material are placed against the end flanges 32 and 34 to shield the coils and to reduce the reluctance of the magnetic flux path.

Iclaim:

1. A differential transformer comprising a' tubular bobbin of a nonmagnetic, nonconductive material, first and second secondary coils of electrically conductive wire wound around said bobbin coextensive over the entire length of the transformer and connected together in bucking series relationship, a primary coil of electrically conductive wire wound around said bobbin over the length of said transformer, said primary coil and said first secondary coil each having a uniformturn density, said second secondary coil having gradually decreasing turn density over the length of said transformer, means insulating said coils, and an armature core disposed within said bobbin for axial movement thereof and having a length less than the axial dimension of said transformer and constructed and arranged to permit the secondary coils to, be magnetically coupled to the primary coil through opposite ends of the armature to achieve null when said core is disposed at one end of'the transformer. i

2. A differential transformer in accordance with claim 1 wherein the turn density of said secondary coils is such that the output voltages of said first and second secondary coils are substantially equal at the null position of said core with the output voltage of said first secondary coil remaining constant while the output voltage of said second secondary coil decreases as said core is moved from the null position.

3. A differential transformer in accordance with claim 1 wherein the turn density of said secondary coils is such that the sum of the voltages of said secondary coils increases in linear relationship to the axial displacement of said core member from the null position.

4. A differential transformer in accordance with claim 3 in which said 'core is within said secondary coils at all times. I

References Cited UNITED STATES PATENTS I 1/1962 Chass 336-136 9/1962 Lipshutz 336-136 6/1964 Persons 336-136 2/1966 Collins 336l36X THOMAS J. 

